The FLPe-mediated circularization of substrate vectors and detection by restriction and Southern transfer. HC-AdFLPe is used to express the site-specific recombinase FLPe that in turn recognizes the Frt sites in the substrate vector and excises and circularizes the DNA fragment between the Frt recognition sites. The circular genomes are 24 to 28 kb in size, depending on the type of substrate vector. HC-AdFLPe remains unchanged. After restriction with HindIII, a 2,200-bp fragment represents the recombined circular genomes of the substrate vector. A 3,300-bp fragment represents unrecombined linear substrate vector genomes, and a 6,800-bp fragment represents the FLPe expression vector HC-AdFLPe. The HPRT probe used here detects an additional cellular DNA fragment of 6,800 bp (not shown).